fixincludes: don't abort() on access failure [PR103306]

Some distro may ship dangling symlinks in include directories, triggers
the access failure.  Skip it and continue to next header instead of
being to panic.

Restore to old behavior before r12-5234 but without resurrecting the
problematic getcwd() call, by using the environment variable "INPUT"
exported by fixinc.sh.

Tested on x86_64-linux-gnu, with a dangling symlink intentionally
injected into /usr/include.

fixincludes/

	PR bootstrap/103306
	* fixincl.c (process): Don't call abort().